linux-kernel - Re: [PATCH v9 01/42] mm: Rename arch pte_mkwrite()'s to pte_mkwrite

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Message-ID: <79b816e7-c331-49f9-058c-02828623ad94@gmx.de>
Date:   Mon, 19 Jun 2023 06:27:41 +0200
From:   Helge Deller <deller@....de>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Cc:     linux-parisc@...r.kernel.org
Subject: Re: [PATCH v9 01/42] mm: Rename arch pte_mkwrite()'s to
 pte_mkwrite_novma()

On 6/13/23 02:10, Rick Edgecombe wrote:
> The x86 Shadow stack feature includes a new type of memory called shadow
> stack. This shadow stack memory has some unusual properties, which requires
> some core mm changes to function properly.
>
> One of these unusual properties is that shadow stack memory is writable,
> but only in limited ways. These limits are applied via a specific PTE
> bit combination. Nevertheless, the memory is writable, and core mm code
> will need to apply the writable permissions in the typical paths that
> call pte_mkwrite(). Future patches will make pte_mkwrite() take a VMA, so
> that the x86 implementation of it can know whether to create regular
> writable memory or shadow stack memory.
>
> But there are a couple of challenges to this. Modifying the signatures of
> each arch pte_mkwrite() implementation would be error prone because some
> are generated with macros and would need to be re-implemented. Also, some
> pte_mkwrite() callers operate on kernel memory without a VMA.
>
> So this can be done in a three step process. First pte_mkwrite() can be
> renamed to pte_mkwrite_novma() in each arch, with a generic pte_mkwrite()
> added that just calls pte_mkwrite_novma(). Next callers without a VMA can
> be moved to pte_mkwrite_novma(). And lastly, pte_mkwrite() and all callers
> can be changed to take/pass a VMA.
>
> Start the process by renaming pte_mkwrite() to pte_mkwrite_novma() and
> adding the pte_mkwrite() wrapper in linux/pgtable.h. Apply the same
> pattern for pmd_mkwrite(). Since not all archs have a pmd_mkwrite_novma(),
> create a new arch config HAS_HUGE_PAGE that can be used to tell if
> pmd_mkwrite() should be defined. Otherwise in the !HAS_HUGE_PAGE cases the
> compiler would not be able to find pmd_mkwrite_novma().
>
> No functional change.
>
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-alpha@...r.kernel.org
> Cc: linux-snps-arc@...ts.infradead.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-csky@...r.kernel.org
> Cc: linux-hexagon@...r.kernel.org
> Cc: linux-ia64@...r.kernel.org
> Cc: loongarch@...ts.linux.dev
> Cc: linux-m68k@...ts.linux-m68k.org
> Cc: Michal Simek <monstr@...str.eu>
> Cc: Dinh Nguyen <dinguyen@...nel.org>
> Cc: linux-mips@...r.kernel.org
> Cc: openrisc@...ts.librecores.org
> Cc: linux-parisc@...r.kernel.org
> Cc: linuxppc-dev@...ts.ozlabs.org
> Cc: linux-riscv@...ts.infradead.org
> Cc: linux-s390@...r.kernel.org
> Cc: linux-sh@...r.kernel.org
> Cc: sparclinux@...r.kernel.org
> Cc: linux-um@...ts.infradead.org
> Cc: linux-arch@...r.kernel.org
> Cc: linux-mm@...ck.org
> Suggested-by: Linus Torvalds <torvalds@...uxfoundation.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Link: https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@mail.gmail.com/
> ---
> Hi Non-x86 Arch’s,
>
> x86 has a feature that allows for the creation of a special type of
> writable memory (shadow stack) that is only writable in limited specific
> ways. Previously, changes were proposed to core MM code to teach it to
> decide when to create normally writable memory or the special shadow stack
> writable memory, but David Hildenbrand suggested[0] to change
> pXX_mkwrite() to take a VMA, so awareness of shadow stack memory can be
> moved into x86 code. Later Linus suggested a less error-prone way[1] to go
> about this after the first attempt had a bug.
>
> Since pXX_mkwrite() is defined in every arch, it requires some tree-wide
> changes. So that is why you are seeing some patches out of a big x86
> series pop up in your arch mailing list. There is no functional change.
> After this refactor, the shadow stack series goes on to use the arch
> helpers to push arch memory details inside arch/x86 and other arch's
> with upcoming shadow stack features.
>
> Testing was just 0-day build testing.
>
> Hopefully that is enough context. Thanks!
>
> [0] https://lore.kernel.org/lkml/0e29a2d0-08d8-bcd6-ff26-4bea0e4037b0@redhat.com/
> [1] https://lore.kernel.org/lkml/CAHk-=wiZjSu7c9sFYZb3q04108stgHff2wfbokGCCgW7riz+8Q@mail.gmail.com/
> ---
>   arch/parisc/include/asm/pgtable.h            |  2 +-

Acked-by: Helge Deller <deller@....de> # parisc

Helge